Series transformer.



G. A. BURNHAM.

SERIES TRANSFORMER.

APPLICATION FILED FEB.8.1911 1,017,731 Patented Feb. 20, 1912.

3 SHEETSSHEET 1.

E] Wuc who 2; GEWMA A BURN/1 G. A. BURNHAM.

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- APPLIQATION FILED FEB.8', 1911. 1,017,737, Patented Feb. 20,1912.

3 SHEETS-SHEET 2.

I $51 Gnome life.

UNITED STATES PATENT OFFICE;

GEORGE AUGUSTUS B URNHAM, OF CLIFTONDALE, MASSACHUSETTS, ASSIGNOR TOSEARS B. 'CONDIT, JR-., 0F BROOKLINE, MASSACHUSETTS.

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Specification of Letters Patent.

Patented Feb. 20, 1912.

To all whom it may concern: Be it known that I, GEORGE A. BURNHAM, acitizen of the United States, residing at Cliftondale, in the county ofEssex and State of Massachusetts, have invented certain new and usefulImprovements in Series Transformers, of which the following is a full,clear, and exact specification.

This invention relates to series transform ers, the object being toprovide a device, of the kind which will maintain its ratio oftransformation more uniform with variation of primary current than thoseheretofore available.

Series transformers are usually employed where working potential is sohigh that bringing the high tension wires to the front of theswitchboard not only complicates switchboard construction, but alsobrings about a condition which is dangerous to They are generally usedto furnish current for operating various indicating or recordinginstruments and for operating trip coils of circuit breakers and otherpro tective devices, the proper operation of which depends mainly uponthe accuracy of the current transformer. I

The transformerconsistsof an irori circuit having a few turns of wirelinking said iron circuit which is connected to the high tension Wiresof the distribution system.

The secondary coil of generally a greater number of turns also links theiron circuit and is entirely insulated from the primary winding. Theratio of transformation is accomplished by varying the relation of thenumber ofprimary or secondary turns.

In the circuit of the primary winding there may be an extremely highvoltage, but in the secondary winding the potential is very low, thevoltage being only suflicient to force the proper amount of currentthrough the indicating or recording ,instruments, thus the dangeroushigh voltage is kept away from the switchboard proper and the lowvoltage employed to actuate the movingelements of the various indicatingor recording instruments.

, The primary winding consists of two sections'which may either beconnected in series or multiple according. to primary load conditionsand in either case within the proper load range of the transformer thesecondary current will be the same. This feature is especially usefuland desirable in keeping down the error in transformation; A furtheradvantage is that a partially loaded feeder circuit may be provided witha double wound current transformer with the coils in series and whenfull load is obtained the coils may be connected in multiple and thesecondary currents will remain exactly the same as in the previous case.Heret-ofore current transformers have had such characteristics that theerror increases rapidly at light load and consequently causes inaccuratereadings in the recording or indicating devices, and furthermore theratio-of transformation and the character of the ration curve ismaterially changed by a variation in the character of-the load on ofthis character. I bring'about this desir-j able result byprovaiding aclosed circuit andpreferably an auxiliary circuit of comparatively lowresistance encircling another part or parts of the magnetic circuit.

The theory of the current transformer is very involved and not generallythoroughly understood, but the favorable results I ob probably .due tothe fact that the efl'ective flux is-nearer constant at all loads thanif the short circuited coil is notemployed. I do not wishto confinemyself, however, to a'ny particular theory regarding the operation ofthis transformer. The fact remains that the series transformer,-constructed" as tain by use ofthe auxiliary closed circuit is I haveindicated, will permit a wide-range of current variation in its primaryand sec ondary without materiallyaltering the'r'a'tio V oftransformation and"further'moreLI able to produce nracticallvthe samecurve with the same ratio of transformationwith P loads of differentcharacterwhichzhasnot been obtained to my lmowledge in the past. Th sisa new; result" in alternating current practice and one of muchimportance to the, switchboard engineer, station operators, con; sumersand producers of electrical energy.

It is also important to note that if any damage occurs to the secondaryof the cmrent transformer a. sudden rise of potential cannot occurbecause of the short circuited compensating winding.

Furthermore my invention aims to protect the current transformer fromhaving high potential surges set up on oc-currei'ice of an open circuitin the secondary which heretofore has been considered as exceedinglydangerous to the distribution system and the apparatus connectedtherewith. From the various figures it will, be noted that with mydevice the transformer is either entirely disconnected from the circuitor the secondary is short circuited if such conditions arise.

My invention, therefore, consists in maintaining the ratio oftransformation of a series transformer by means of an auxiliary closedelectric circuit of low resistance around its core which comprises meansfor automatically protecting the transformer in case its secondarycircuit rises unduly in voltage.

The several features of novelty will be hereinafter more fully describedand will be definitely indicated in'the claims.

In the accompanying drawings which illustrate my invention Figurel is asectlOIlfll" view of a transformer embodying a compensating coilaccording to my improvement. Fig. 2 is a diagrammatic view of a modifiedimprovement incwhich part of the secondary is closed on itself to formthe compensatin coils." Fig. 3 showsa compensating coil of variableresistance. Fig. 4 shows a variable adjustment ofthe compensating coilby means of inductance. Fig. 5 shows means for preventing the transformer fromdamage by automatically cutting it out of circuit. Fig. 6shows means for closing the secondary and protecting the transformer incase its secondary be-' comes opened and for cutting out the transformerif the trouble is not automatically corrected. Fig. 7 shows the relativeefii: ciencyin maintainingthe ratio of transformation of my improvedtransformer and I those heretofore employed. Fig. 8 shows how mytransformer may be connected for a different load. Fig. 9 shows curvestaken 1 with the same transformer with and without the compensatingcoil. i 'The load circuit of a series transformer commonlydncludesindicating and recordingammeters and watt meters and trip coils"ofcircuit breakers involving particularly in d sease of'recordinginstruments a variable fiload; moreover the number of these instrum'entsand therefore the load is widel ferent on different swi-tchboards analsodif-.

differs from time to time when instruments are temporarily out out ofservice or their circuits are varied, and as series transformers are astaple article of production the same device does not properly meet thevarying uses it is put to. lVhen the primary load is heavy it is usualto connect the two primary windings in parallel and when light inseries. Such a provision does not however satisfy the condition ofmaintaining the ratio constant under changes 'of load, especially ifpartly or wholly of an inductive character, the ratio being altered witheach load variation so that the secondary voltage is not uniform andtherefore the measuring instruments no longer read correctly and thetrip coils no longer act according to their predeterminedcalibration.The error is easily seen by an inspection of the curve A of- Fig. 7which shows .cent. from small to full load. This is a serious defectasthe measuring instruments are no longer reliable in their indicationsand the change is so great that if the series trans former is applied totrip coils they do not respond to the calibrated loads. With mystructure, however, the ratio is preserved practically constantthepercentage of error being reduced to about one per cent. for loadchanges from 30 to 100 er cent. and over as will be seen from curve B.These two curves represent a comparativetest of a standard series.transformer and oneconstructed according tomy improvements.

As indicated onlFig. 9 the numerals on the axis of ordinates atthe leftof the diagram represent the true ratio divided by the marked ratio asnoted" at the top of the diagram which really indicates the percent- 55*,represents' a'closed magnetic circuit of laminated iron held by ironclamps 6 6" and supported on a. base 7. The core 5 is insulated and a.secondary winding 8 wound on theinsulation Another layer of insulationis then applied over the secondary winding and the primary windin 9 thenapplied on the outside This may e inthe form of two superposed layerseach having terminals 10, 10? which may be connected in series rx muu.The drop of potential over the primaryfwinding is small andthe'consecutive turns need not be insulated to any great amount. Inpractice in the case of a 10 or 20 to 1 ratio: of transformation I makethe primary of two layers of fifteen turns each and give the secondary240 turns. The compensating coil which comprises a 5 tertiary circuit isindicated at 11.. This is of low resistance and may be variouslyconstructed. I have attained excellent results with 15 turns ofcomparatively coarse wire ,.,say No. 16 B. & S. gage; it might be ofdifferent resistance, a closed copper tube might be employed in somecases, or the resistance might be varied inductively or non-inductively,or in some cases a part of the secondary might be short circuited onitself as shown at 12 in Fig. 2.

Fig. 3 shows diagrammatically a method of non-inductive variation of thecoil re sistance a resistance 13 being connected in the compensatingcircuit and an adjustable contact 14 being provided to vary the point ofconnection with the resistance and thus alter the current in thecompensating winding. In- Fig. 4 a coil 15 is included in circuit withthe compensating coil and a movable iron core mounted within the coil 15to alter the impedance of the compensating circuit. The compensatingcoil may be used to supply a trip .coil directly as shown at 17 in Fig.5, the secondary coil being used as 80 usual on the measuringinstruments as shown at 18 in said figure or the secondary may supplyboth the measuring instruments and the trip coils as shown in Fig. 6. Inthe former case the trip coil will act to cut out the series transformerby opening the circuit breaker 19 in case of damage to the sec ondarycircuit of the transformer, since any substantial change of resistanceor a break in the secondary circuit will increase the current in thetrip .coil and cut out the transformer. Thus the transformer is savedfrom being burned out by excessive increase of voltage in its secondaryand resultant break down by overheating.

In Fig. 6 I have shown an arrangement by which trip coils on theswitchboard may be used to open the circuit in case of over load in theprimary circuit and may also be protected from excessive rise of voltagein the secondary of the transformer. In

this case the secondary winding may be placed in circuit with the tripcoil and if desired with other load, and a small relay 20 may be put inthe compensating circuit; if the transformer primary is excessivelyloaded the increased current in the secondary will cut it out byoperating the trip coil and opening the circuit breaker 21; or if thereshould be a'break of the secondary circuit or any other trouble thatwould create an improper rise of secondary VOltage, the relay will closethe secondary circuit and prevent damage to the transformer. Referringto Fig. 9, I have shown two sets of curves, namely, C D and E F; the twoformer curves taken from a transformer using my compensating coil andthe latter atransformer omitting the compensating coil] Both of thesetransformers were operated on a load of the same character and acomparison of the two curves will show the advantage of the compensatingcoil. I

Curve C shows the ratio of transformation with the trip coil in thecircuit. Curve D shows the ratio of transformation with the trip coilout of circuit. It will be noted that these two curves are parallel,there being practically no error in slope at corresponding pointsthroughout the total length of the curves.

Curve E shows the ratio of transformation of the same transformerwithout the compensating coil with the trip coil in the circuit andcurve F shows the ratio. of transformation with the trip coil out of thecircuit. Comparing curves E and F it is easily seen that the characterand also the ratio of transformation of the two curves are greatlyeffected by the'varying load conditions.

The numerals on the axis of ordinates at the left of the diagramindicate the theoretical ratio and those on the axis of the abscissaethe percentage of full load, the variation from the horizontal showingthe change of ratio under the difierent operating conditions described.

Fig. 8 illustrates a device whereby the curves of ratio can be made tocoincide when it is desiredto make any change in the character of thesecondary load of the current transformer.

22 is a primary winding of the current transformer; 23, compensatingcoil; 24', the secondary winding linking a magnetic circuit, 27. It willbe noted that a tap (25) is brought out from the secondary (24) and alsoa tap (26).

The curve C of Fig. 9 was obtained by connecting the load, consisting ofinstruments and relay, to the tap 25/ With the coil disconnected thecurve of transformation is shown in D. By connecting this same load tothe tap, 26 the curve D becomes G and it will now be evident that bythis type of transformer we may obtain a curve which is nearer thetheoretical ratio line and we may also produce the same curve forvariations in the character of the load, as well as for difl'erence inthe current values of loads of the same character.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is V 1. A series transformer having a primarywinding, a secondary winding, and a closed circuited coil to preserveits ratio of transformation t'mder change of load.

2. A series transformer having a prim' winding, a secondary winding, andan a justable short circulted compensating coil 1 4: I mirror topreserve the ratio of transformation of the transformer. v

v 3. A. series transformer having a primary Winding, a secondarywinding,a closed eirtranefoirmer having prinnir r windings ant a closed oirrWinding, whereby the ratio tion under change of eeool'ntair loa.etically uniform;

it series transformer provided With means for preserving its ratio oftransforo trait rte ma rnetic oircnin and mation under Wide ranges ofsecondary load variation comprising a closed circuited anailiary coilaround its magnetic circuit.

7. A; series transformer provided with a short eirenited compensatingcoil to main tain the effective magnetic flux approat mately constantover a Wide range of secondary load variation, whereby the ratio 30 oftransformation is preserved practically constant,

dqeeriea transformer provided. with a closed eirenlted eompensatin coilaround in provided with a plurality of tape to t n erent loader intestimony whereof lt aitia' my signature in presence of two WIlmKJSSES,

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eecondary Wind-

